Science Centres: Climate

Tropical rainfall outlook: August to October 2008

Rainfall outlook map for August to October 2008. (Click for enlargement)

Sea surface temperature outlook map for August to October 2008. (Click for enlargement)

La Niña conditions have dissipated in the equatorial and subtropical southwest Pacific. During the August–October 2008 forecast period, a large region of suppressed convection is likely to encompass Tuvalu, Tokelau, the Northern Cook Islands, Tuamotu, the Society Islands, Pitcairn Island, and the Marquesas, with average–to–below or below average rainfall expected for those countries.

Enhanced convection is likely from Papua New Guinea extending in a band southeast toward Vanuatu and through Tonga, with above average rainfall expectedin those countries. Near–to–above average rainfall is forecast for Fiji, Niue, the Solomon Islands and New Caledonia for the coming three-month period.No clear precipitation guidance is offered for Eastern Kiribati, Wester Kiribati, the Southern Cook Islands or the Austral Islands.

SSTs are expected to be normal to above normal near Papua New Guinea, and within a region extending from Vanuatu southeast to the Austral Islands and Pitcairn Island. Fiji, Tonga, and Niue are expected to have above average SSTs. Below normal SSTs are forecast for the region extending from Tuvalu southeast to the Society Islands, while normal to above normal SSTs are forecast for Western Kiribati and the Marquesas Islands.

The confidence in the forecast model skill for this seasonal rainfall outlook is moderate to moderately high for most Pacific Island countries. In the past, the average region-wide hit rate for forecasts issued in August is 58%, 3% lower than the long-term average for all months combined. The SST forecast confidence is moderately-high to high.

NOTE: Rainfall estimates for Pacific Islands for the next three months are given in the table. The tercile probabilities (e.g., 20:30:50) are derived from the interpretation of several global climate models. They correspond to the odds of the observed rainfall being in the lowest (driest) one third of the rainfall distribution, the middle one third, or the highest (wettest) one third of the distribution. On the long-term average, rainfall is equally likely (33% chance) in any tercile.